U.S. Pat. No. 4,212,153 to Pedicano and U.S. Pat. No. 5,107,470 to Pedicano and Sacks disclose time-delayed ink message display systems which are multilaminate labels composed of a set of upper layers and lower layers, as shown in FIG. 1 of the accompanying drawings. The upper layers comprise at least a clear polyester protective film through which the message may be seen, an opaque pressure sensitive adhesive migration layer for attaching the upper layers to the lower layers, and a release liner to prevent activation of the system. Additional layers may be added to the upper layers, as shown in FIG. 1 herewith. Addition of a polymer film, with another layer of pressure sensitive adhesive, may be included to slow the dye migration and extend the amount of time required for the message to appear through the Mylar film. The lower layers consist of, at a minimum, ink printed onto a polymer substrate, which is typically polyester. The system is initiated by removal of the release liner and attachment of the upper layers to the lower layers. Over time, the dye in the ink diffuses through the upper layers and gradually becomes visible to the human eye as the ink reaches the top. The ink may be printed onto the substrate such that a message appears after some time. Such messages may appear in the form of words or a picture meant to have a specific and evident meaning.
Time-delayed message systems have a wide variety of applications especially in the medical field. For example, self-administered medical diagnostic kits can require the careful monitoring of time. Since people's perception of time varies drastically, a time-delayed message system which produced the word "STOP" at a predetermined time would be of great value and add significantly to the efficacy of diagnostic kits. Other medical applications include use of the message systems as a timing device to remind patients to take their medications, or to change a transdermal drug delivery patch after a predetermined amount of time.
Known time-delayed message systems, shown in FIG. 1, have the characteristic that the dye, and hence the message, gradually appear at the top of the upper layers over time. When the system is first activated, the message is not immediately visible. As the dye diffuses through the upper layers, the message slowly begins to appear. At first, the message is hint and unreadable. As the message becomes slightly stronger, some individuals can read it while others are cannot. The more time that passes, the stronger the message becomes and the more people can read it. Eventually, the message becomes sufficiently strong and clear as to be readily readable. A fundamental characteristic of such systems is the gradual, time-related development of the message. This presents a practical limitation of message display systems in that the amount of time which the message is delayed is ill-defined and can vary considerably from person to person due to differences in individual perception, (A quantitative example of the gradual nature of these systems will be described below). The slow visual development of known systems is a result of the inherent mass transfer properties of solutes diffusing through matrices. In this context the message ink or dye is a solute and the migration layer of adhesive, optionally with a polymer layer, is the matrix. A principle of physics, Fick's law states that the rate of solute diffusion through a homogeneous slab will be constant during steady-state conditions. Frick's Law presents a problem in attempting to accelerate the visible message development phase of a time-delayed migrating ink message. The physical limits of film thickness necessary to retain structural integrity and opacity (without which the message image printed on the lower layers will be visible at all times), impose a significant time limit on the development phase during which the displayed image develops from a perceptible contrast area to a recognizable message or image. Consequently, the message image may be visible in a shadowy or embryonic form before it is legible, and may be unduly slow to develop, providing a confusing or unaesthetic effect. The message display system lacks "focus". In most cases, the message will be time-sensitive, for example to indicate a time to take a medication, and its slow development will cause serious uncertainty.